The present invention relates to a method and apparatus for manufacturing aluminum extrusion dies. Typically, such dies comprise circular steel discs with an aperture or void in the center which has a cross-sectional shape corresponding to the desired cross-sectional shape of the aluminum extrusion to be produced. A bar of aluminum is drawn through the die aperture so as to force the aluminum to take the cross-section of the aperture.
The steel employed in the mold must be of a very hard, non-porous type. Any porosity in the die aperture of the mold will create lines on the extruded part. Worse, the porosity may create hidden defects which show up only after the aluminum extrusion has been anodized. The extruded piece would thus be put through additional manufacturing processes and expense before the defect would even show up.
Prior artisans have manufactured such extrusion dies from work-hardened steel bar stock of ASTM H-11, H-12 or H-13 steel. While other steel chemistry types might be better suited to aluminum extrusion dies, H-11, 12 and 13 steels are the best presently available. A steel specifically adapted to the extrusion die industry is not justified in work hardened bar stock lots by the quantity required.
The bar stock has a diameter corresponding to the die size needed for the particular part being extruded. Typically, the bar stock and the resulting die must have a 7 to 25 inch diameter. The bar stock is then sliced into discs of about one inch thickness. An electrical discharge machine is used to machine the desired die aperture in the center of the disc.
Such dies are then used until the die aperture is worn and they are then disposed of. While the original steel bar stock costs about one dollar a pound, the scrap value of used dies is only one to five cents per pound. Prior artisans have been unable to recast these dies into die blanks because the resultant blanks have too much porosity. The scrap dies cannot even be recycled specifically as H-11, H-12 or H-13 steel because the steel manufacturers cannot get enough of them regularly to pour the large quantities required to make a "pour" economical. This further contributes to the lack of value which the used die blanks have as scrap steel.
Some attempts have been made to melt down used dies and recast them as aluminum extrusion die blanks. To date, these attempts have not been commercially successful. One problem has been the difficulty of avoiding porosity in the die blank. This is particularly true when the blanks are cast without any void or aperture in the center. The porosity problem was somewhat lessened in situations where a permanent core, having the desired cross-sectional shape, was used in the casting process, but the difficulties encountered utilizing conventional molding techniques were still not completely overcome. Further, the use of a permanent core elevates the tooling cost. The tooling costs for conventional molding processes require that a substantial number of extrusion dies be made having the same shape, including the same die aperture shape. Unfortunately, manufacturers don't need a large multiplicity of dies having the same given shape. In contrast, a variety of different die aperture shapes are generally required for the variety of different types of extrusions which are manufactured.
Accordingly, prior artisans have continued to manufacture their aluminum extrusion dies from steel bar stock and the scrap value of the worn dies has been almost negligible. Attempts to re-melt the dies and cast them directly into die blanks have been commercial failures.